Nuclear astrophysics deep underground : the LUNA experiment

Nuclear reactions that generate energy and synthesize elements take place inside the stars in a relatively narrow energy window: the Gamow peak. In this region, which is in most cases below 100 keV, far below the Coulomb energy, the reaction cross-section drops almost exponentially with decreasing energy. The extremely low value, from pico- to femto-barn and even smaller, has always prevented its measurement in a laboratory at the Earth’s surface, where the signal-to-background ratio would be too small because of the background generated by cosmic-ray interactions. Therefore, the observed energy dependence of the cross section at high energies is usually extrapolated to the low-energy region, leading to substantial uncertainties. In particular, a possible resonance in the unmeasured region is not accounted for by the extrapolation, but it could completely dominate the reaction rate at the Gamow peak. The LUNA (Laboratory for Underground Nuclear Astrophysics) collaboration has exploited the low-background enviroment of the Gran Sasso National Laboratories in Italy to measure nuclear reactions that take place in stars at astrophysically relevant energies, at or close to the Gamow peak. The most recent reults obtained at LUNA will be presented and an outlook on ongoing and future projects will be given.